Apparatus for processing electromagnetic wave energy

ABSTRACT

A device for processing electromagnetic wave energy by converting the wave energy to spin waves and/or elastic waves, the present disclosure being particularly directed to the concept of providing a device made up of a single-crystal material which contains non-uniform material parameters to give graded values of material saturation 4 pi Ms. The material discussed in greatest detail is YIG and the non-uniform material parameters are furnished by doping the YIG with gallium. The graded material parameters in combination with an external magnetic bias field H result in an internal magnetic bias field H. The electromagnetic energy as it enters the material is acted upon in a manner that is influenced greatly by the contour of H. The contour of H in the present disclosure, in turn, is predetermined to present gradations in H which will allow the conversion mentioned and which allow predetermination of the wavenumber k of the magnons and/or phonons thereby formed. Furthermore, the place or space within the crystal at which conversion occurs can be somewhat determined by the profile of H thereby provided.

United States Patent 1 Morgenthaler July 15, 1975 APPARATUS FORPROCESSING ELECTROMAGNETIC WAVE ENERGY [73] Assignee: MassachusettsInstitute of Technology, Cambridge, Mass.

[22] Filed: Feb. 7, 1974 [21] Appl. No.: 440,317

Related US. Application Data [60] Division OfSer. No. 173,884, Aug. 23,1971, Pat. No. 3,811,941, which is a continuation of Ser. Nos. 740,751,June 27, 1968, Pat. No. 3,609,596, and Ser. No. 645,947, June 14, 1967Pat. No. 3,540,302.

[52] US. Cl 333/30 M; 333/241; 333/242; 333/71 [51] Int. Cl. H03h 9/22;I-lO3h 9/30; l-l03h 9/34 [58] Field of Search..... 333/241, 24.2, 24 G,30 M, 333/31 R, 21 A, 71; 252/6257, 62.58;

[56] References Cited UNITED STATES PATENTS 3,085,981 4/1963 Tanenbaum252/6257 Primary Examiner,lames W. Lawrence Assistant Examiner-MarvinNussbaum Attorney, Agent, or FirmArthur A. Smith, Jr.; Robert Shaw;Martin M. Santa [5 7 ABSTRACT A device for processing electromagneticwave energy by converting the wave energy to spin waves and/or elasticwaves, the present disclosure being particularly directed to the conceptof providing a device made up of a single-crystal material whichcontains nonuniform material parameters to give graded values ofmaterial saturation 41rM,. The material discussed in greatest detail isYIG and the non-uniform material parameters are furnished by doping theYIG with gallium. The graded material parameters in combination with anexternal magnetic bias field l-l result in an internal magnetic biasfield H. The electromagnetic energy as it enters the material is actedupon in a mann'er that is influenced greatly by the contour of H. Thecontour of H in the present disclosure, in turn, is predetermined topresent gradations in H which will allow the conversion mentioned andwhich allow predetermination of the wavenumber k of the magnons and/orphonons thereby formed. Furthermore, the place or space within thecrystal at which conversion occurs can be somewhat determined by theprofile of H thereby provided.

2 Claims, 12 Drawing Figures ///l. l////////////////] .'V///////j//////// l/////l.' 'V////l ////////N PATENTEDJUL 15 ms SHEET 1 WAVEAMPLITUDE FIG. 2

PATEmEnJuL 15 ms 3695324 RELATIVE EFFICIENCY FIG. 4A

FIG/1B APPARATUS FOR PROCESSING ELECTROMAGNETIC WAVE ENERGY Theinvention herein described was made in the course of contracts with theOffice of the Secretary of Defense. Advanced Research Projects Agency.and Air Force Cambridge Research Laboratories, Office of AerospaceResearch.

This application is a division of application Ser. No. 173.884 and isbeing filed as a result of a requirement for restriction in the earlierapplication. Application Ser. No. 173.884 now US. Pat. No. 3.811.941 isa continuation of application Ser. No. 645.947 filed June 14, 1967 (nowUS. Pat. No. 3.540.302) and application Ser.- No. 740,751, filed June27. 1968 (now U.S. Pat. No. 3.609.596) and was filed in response to arequirement for restriction in each instance.

In an application for Letters Patent entitled. Method of and Apparatusfor Changing Frequency, Power and/or Delay Time of Wave Energy. Ser. No.645,947. filed June 14. 1967 by the present inventor (now US. Pat. No.3.530.302). there is described apparatus and method for convertingelectromagnetic wave energy to propagating spin wave and/or elastic waveenergy and vice versa. It is noted therein that under appropriatecircumstances the conversion from one to the other form of energy can bemade and the angular frequency (w) and wavenumber (k) of certainpropagating wave energy can be changed by changing the environmentalparameters within the material. in an article entitled. Photon/MagnonConversion Near A Material Interface. in Electronics Let/ers. July 1967.Vol. 3, No. 7. pp. Z99302. the present inventor discusses delay meanswhereby substantial time delay of wave energy is obtained even in thinfilm materials and in a relatively non-dispersive fashion. Thus, it hasbeen found that under appropriate circumstances an electromagnetic wavepropagating in air may be presented. for example. to a magnetic materialand can be converted to evanescent wave energy and propagating spin waveenergy within the material. Whereas the spin wave propagates within thematerial. the evanescent wave is stationary and degeneratesasymptotically spatially. there being an interchange of energy betweenthe evanescent wave and the propagating wave which results in evanescentwave energy being transferred to the propagating spin wave and viceversa.

The apparatus hereinafter described is adapted to provide time delay ofelectromagnetic wave energy: but. in addition. it provides filtrationand/or switching as well. It has been found, moreover. for presentpurposes, that although there can be no conversion at the interfaceboundary from electromagnetic power transfer in air to quantummechanical exchange power transfer by the propagating spin wave in thematerial. nevertheless. under certain conditions, conversion can occurnear the boundary. Conversion from conventional electromagnetic waveenergy to spin wave energy. as noted in US. Pat. No. 3.530.302. may beeffected by preseting a graded internal magnetic bias field to theelectromagnetic wave or by adjusting the level of the bias lield to oneat which conversion can take place. In the present case a delay materialis chosen that is adapted to present an abrupt discontinuity to theelectromagnetic wave energy. thereby to proide photon-to-magm nconversion very near the material interface. the level of the externalmagnetic bias field being adjusted to an appropriate level to effectsuch conversion. The direction of propagation of the spin wave and thedirection along which the evanescent wave degenerates is determined, ashereinafter dis cussed in greater detail. by the direction of the incoming wave into the magnetic material and the direction and magnitude ofthe internal d-c magnetic bias field. It is also possible. using theprinciples taught herein. to control the direction of spin wave powerflow in the material and to change that direction very quickly by, forexample. small changes in the d-c bias field. Accordingly. a principalobject of the present invention is to provide apparatus and methodadapted to switch a microwave electromagnetic signal from. for example.one output to another in microseconds by providing apparatus that isvery sensitive to changes in the direction and magnitude of an internald-c magnetic field, provision being made for changing the magnitude and/or direction of the field.

1t has been found that if the wavenumber of the spin wave at conversionis at a value herein designated k in the ark diagram, there is a maximumtransfer from evanescent wave energy to spin wave energy, and. at thatvalue, approximately 50 percent of the spin wave power is carried ineach of the electromagnetic channels and the quantum mechanical exchangechannels. Also. k represents the value at which the group velocity ofthe propagating spin wave, in the absence of transverse boundaries, isminimum so that delay is maximized and at which dispersion is alsominimum so that the integrity of the input wave is maintained. Therefore. the delay material is chosen. and the external magnetic bias isadjusted to provide an appropriate environment to effect conversion nearthe interface at values of wavenumber k which result in the desireddelay and allowable dispersion for the particular application. Hencc.another object ofthe prescent invention is to provide a method of andapparatus for providing substantial time delay of electromagnetic waveenergy while nevertheless preserving substantially the wave shape of theinput energy.

A further object is to provide such substantial time delay even in athin film material.

Another object is to provide a delay means for electromagnetic waveenergy in which the energy is subjected to one or more abruptdiscontinuities by providing a magnetic material consisting of one ormore layers or regions to provide the discontinuities.

1n delay apparatus for which the present invention is adapted. it isimportant not only that an appreciable delay time be provided in a smallvolume; but it is also of importance that the delay time. onceestablished, be substantially constant. Since. in equipment, theenvironmental parameters presented to the wave energy will change due.for example. to temperature changes. electric current fluctuation to thecoils that provide the external bias field, and the like, it is to beexpected that changes in delay time ofthe wave energy will occur;accordingly, an additional object of the present invention is to provideways for reducing the effect upon delay time of uncontrollable changesin the equipment.

A still further object is to provide apparatus in which the time delayof the wave energy is related to the magnetic field thereby to give anindication of the magnitude of the magnetic field.

Another object is to provide apparatus adapted to remove or filterundesirable frequencies of the wave energy.

Other and further objects will become evident iirthc specification tofollow and will be more particularly pointed out in the appended claims.

The objects of the invention are attained, broadly, by a method thatcomprises, introducing electromagnetic wave energy to a material thatwill support propagating spin wave energy and evanescent wave energy andallow interchange of energy therebetween, the material being one inwhich the electromagnetic wave energy can be converted to evanescentwave energy and propagating spin wave energy and in which the groupvelocity of the propagating wave and the rate of decay of the evanescentwave can be varied by varying the environmental parameters within thematerial. The material parameters are such that, when subjected to anappropriate applied magnetic field, an abrupt discontinuity isencountered by the electromagnetic wave thereby to effect the conversionof the wave energy from electromagnetic power to a combination ofevanescent and spin wave, the spin wavepower being carried in theelectromagnetic channel, the quantum mechanical exchange channel or acombination thereof.

The invention will now be explained in connection with the accompanyingdrawings in which:

FIG. I is a sketch representing a wave amplitude pro: file showing inputand reflected electromagnetic wave energy at an air-material interfaceof a magnetic material and evanescent and spin wave energy within thematerial;

FIG. 2 is a sketch representing the intrinsic relationship betweenangular frequency (w) and wavenumber (k) of the spin wave and decay rate(a) of the evanescent wave in the material in the absence of transverseboundaries and for the special case in which the angle ill defined belowequals zero and the wave energy is circularly polarized:

FIG. 3 is a graph ofthe relative conversion efficiency of power fromelectromagnetic wave energy in air to electromagnetic spin wave energyand exchange spin wave energy in the magnetic material as a function ofn, where n =k/k k ,./0z and where k is a point on the w k and w a curvesin FIG. 2 at which k a:

FIG. 4A is a vector diagram to show the group velocity (V,,) of a spinwave as a vector sum of the parallel component (V,,) n and theperpendicular component (V!!) .L

FIG. 4B shows the vector relationship between the wave vector? and thegroup velocity vector (V when there is some angle lIJ between ITand aninternal magnetic field designated H, and Cartesian coordinates y, 1'are also shown; I

FIG. 5 is across sectional view of a magnetic material adapted toperform the functions herein disclosed;

FIG. 6 shows the material of FIG. 5 in reduced size and also,schematically, shows means for providing a z-directed internal d-cmagnetic field with further means for rotating the field through angles+t11 and -11: to the z-direction;

FIG. 7 is a modification of the apparatus of FIG. 5

yttrium iron garnet [.YlGl)v in the disclosed embodirnent witha firstnon-magnetic metal reflection wall [4 (which may be a thin film ofsilver, aluminum, gold or the like) secured toone face thereof and asecond metal reflection wall 13 secured to the other face. An inputaperture or window 24 admitselectromagnetic energy to one region of themagnetic material 12 and output apertures or windows 25 and 26 allow theenergy to be withdrawn from the material at another region thereof.Output apertures other than 25 and 26 may be provided, and the relativelateral positions of 24, 25 and 26 may be changed, the wave energy beingdirected to one or the other of the windows in a manner explainedhereinafter. The material is one that will support propagating spin waveenergy and evanescent wave energy; and is, further, one in which theelectromagnetic wave energy can be converted to evanescent and spin waveenergy and which allows interchange of energy between the evanescentwave and the spin wave. In addition, the group velocity (V,,) (the pathof travel of power in the spin wave, as represented by ci ther of dottedlines 18 and 19 in FIG. 5, is the same direction as the group velocityvector) of the spin wave and the'rate of decay (oz) of theevancscentwave can be varied by varying the environmental parameterswithin said material. A suitably doped yttrium iron garnet crystalserves the foregoing purposes. The material parameters of YIG can bevaried by different types and amounts of dopant (See an articleentitled, Magnetic Effects of Indium and Gallium Substitutions inYttrium Iron Garnet, Anderson ct al,, Journal oft/1e Physical Societyofjupan, Vol. 17, Supplement B-l, Proceedings of the Conference onMagnetism and Crystallography, September 1961 and the materialparameters in combination with an external applied d-c magnetic fieldcompose the environmental parameters (i.e., the internal d-c magneticbias field) encounteredby the wave energy within the material. The valueof the magnetic field and the magnetic characteristics of the particularmaterial used are chosen to provide at least one abrupt discontinuity inthe equilibrium environmental parameters to effect the conversion fromelectromagnetic power to a combination of evanescent and propagatingspin waves, the spin wave power being carried in the electromagneticchannel, the quantum mechanical exchange channel or a combinationthereof.

It is in order now to discuss the theory of the concepts hereindisclosed. The discussion in the Morgenthaler article relates to thespecial case of a positively circularly polarized electromagnetic wavein air at normal incidence to the interface boundary between air and alossless material, as a lossless material 12. In the special case theangles ill and [3 discussed below both equal zero. The discussion tofollow, however, relates to the general case where t]; and [3 need notequal zero and, in fact, cannot equalzero if steering of the propagatingspin wave is to. be effected. The angle B, as shown in FIG. 4B, is theangle between the lTvector of a pin wave and the group velocity vectordesignated (V,,) whenthe internal bias field H isat some angle ill tothe Fvector. A number of expressions may be used to define the angle [3,as discussed later, one such expression being the following:

where k is the wavenumber of the propagating spin wave; A is theexchange constant (approximately 3 l()" cm for YIG m is a constant 1r Xl(]' rad/- sec for YIG). w I yla.,H where y is the gyromagnetic ratioand ,u... is the permeability of free space (w 211' X 2.8 X X H (oe.)rad/sec for YIG): (0/0. where is the velocity of light and equals 3 X10' cm./sec. and w to The expression shown at l does not includemagnetic anisotropy and is valid if M k,, /k-. The following expressionis valid if \I) l and it k The expression (2) includes higher ordercorrections to the magnetostatic approximation and. again. does notinclude magnetic anisotropy. The exact expression for tan ,8 has theproperty that as k It ,8 independent of Ill.

The following analysis (expressions (3) through (9)) is carried out inthe magnetostatic approximation. is valid when magnetic anisotropy ispresent and up proaches the exact solution for large values of k, i.e..k k:

where X IT,, is the time-averaged electromagnetic power flux (watts/m Zis the electric field (volts/m IT is the'magnetic field (amperes/m.). aand b are the amplitudes of the small signal (s.s.) magnetization vector "2., (amperes/m.) (u h is a circularly polarized wave and a b in anelliptically polarized wave).l and 7;, are unit vectors in Cartesiancoordinates. 7' Z'x' T 'y 71:. r is time (seconds). ,u.,, A 517/8F6m/6Iis the quantum mechanical exchange power flux (watts/m W is thetime-averaged energy density of the wave (joules/m w is I 'y|p.., M M..is the saturation magnetization (amperes/m). Ts total time average powerflux (watts/m and (V,,) n and (V .t are respectively the components of(V,,) parallel and perpendicular to the A" vector. From the foregoingexpressions it can be seen that the path (18 or 19. for example. in FIG.5) taken by the propagating spin \vave within the material 12 can bechanged in a number of ways. It is possible to perform a switchingfunction by. for example. tilting the E field from +tl1 to -d; in FIGS.5 and 6 to switch the output from the window to the window 26 (az-directed field in FIG. 6 can be provided by poles 2] and 22 andchanges from +tIJ to -11! can be effected by the coil shownschematically at 23). or the external bias field can be changed inmagnitude to effect changes in 6. I-urtbermore. as will be explainedmore fully hereinafter. the delay time ofthe wave energy within thematerial 12 can be varied by changing the internal bias H in magnitudeand/or direction. and the frequencies that will reach the ports 25 and26 along paths l8 and 19, respectively. can be affected in like mannerto accomplish a filtering function. However. before any work can beperformed. upon a spin wave. a spin wave must be created. The moregeneral explanation of conversion from electromagnetic power to spinwave power contained in said U.S. Pat. No. 3.530.302 is summarized belowand amplified in some particulars. The circuit arrangement to providethe results discussed can be that shown in FIG. 7, where a magneticmaterial is shown disposed within a waveguide 65.

A profile representation of the wave energy previously discussed may bethat shown in FIG. I. where l represents incoming electromagnetic waveenergy traveling in air. which enters a material and converts toevanescent wave energy (the decay rate of which is represented by thecurve shown at 2) and propagating spin wave energy (represented by thesinusoid shown at 3 (In FIG. 1. at the air side of the interface. inaddition to the incoming wave 1. there is shown a similar but oppositelydirected'wave 4 to represent reflected electromagnetic energy.)

The ordinate in FIG. 1 can be taken as an interface representing. forexample. air to the left and some magnetic material 60, as yttrium irongarnet or other material to the right. It is pointed out in theMorgenthaler article that when the wavenumber k of the spin wave duringconversion is equal to some value k.,. in FIG. 2 (n k/l l the groupvelocity of the spin wave is minimum. resulting in greatest delay timein the material; and, also. dispersion is minimum. Moreover. at k thecurves to k and w a coincide.

Some of the electromagnetic energy represented by the curve 1. uponentering the delay material. proceeds axially (in the z, direction)within the material and through. This represents a smalLamount of theinput energy. and it passes along out of the material. with some delaytime. It is propagating wave energy and may be termed a precursorbecause it serves the useful function of preceding the energy to comebehind and begins the establishment of the evanescent wave. At or nearthe interface an evanescent wave forms which decays in magnitude. asrepresented by the curve 2. The rate of decay a can be modified. forexample. by changing the internal magnetic bias H. to the value shown.for example. at 2; the particular condition depicted by 2' results froman increase in the internal magnetic bias field H. (The change in H maybe effected by changing the strength of the external field or byproviding appropriate material graduations by doping the YIG mate rial.as discussed in the Anderson et al. article.) In FIG. 2 the w k curvesshown at 10 and 11. representing spin waves and electromagnetic waves.respectively. and the w "d curve shown at 9 of the evanescent wave willall shift upward as a result of the increase in H. and k would shift tothe right of the position shown. the shift reflecting the increase indecay rate a. represented by the curve 2. and the increase in wavenumberk. represented by the dotted curve shown at 3. An appropriate value ofexternal magnetic bias can be applied. therefore. to give the desiredpoint of conversion to fit a particular requirement.

A more complete explanation of the method of power transfer in the spinwave will now be made. The discussion is restricted to a situationwherein a circularly polarized electromagnetic wave in air is introducedto a delay material at the air-tomaterial interface and the wave, uponstriking the face, does so along a line that is orthogonal to said face;i.e.. the direction of propagation is axial (;-directed in FIG. 7) andthe faces of the delay material, as the material 60. are perpendicularto said direction. Furthermore. a .i-dirccted external d-c magnetic biasfield of sufficient magnitude to saturate the material is used. In sucha circumstance the group velocity of the spin wave (V,,) is equal to theparallel component (V II thereof. but does not equal (V except in thisspecial case.

Spin waves refer to energy waves appearing in a material by virtue ofprecessional movement of electrons about a magnetic axis. Energy can bepumped into the spin wave or removed therefrom. The dipoles formed bythe precessional movement pass energy to adjacent dipoles by quantummechanical exchange means or by electromagnetic means. Either way.however. the frequency of the wave bears a one-to-one relationship withthe precessional movement of the dipoles, and,,indeed. there is at alltimes some electromagnetic power transfer mixed with some exchange powertransfer within the spin wave. Since electromagnetic power tends totravel fast. at the speed of light in air but at reduced speed inmagnetic materials, and the exchange power travels relatively very muchslower. the speed at which power transfer takes place within thematerial depends upon the relative mixture of the forms of powerappearing in the spin wave. At values of II l. the conversion of powerto the electromagnetic spin wave channel, as shown by the curve shown at6 in FIG. 3, is much greater than the conversion to the quantummechanical exchange channel of the spin wave, as represented by thecurve shown at 5. (The combined efficiency is represented by the curveshown at 7.) The parallel component as a function of n is (l-,,)-n"-l-l/2n and rises dramatically for values of n l. (The perpendicularcomponent. though not being considered at this juncture, is also afunction of n, in the magnetostatic approximation. bearing therelationship (V,,) 1- l/n.) Thus the values of k and a at whichconversion takes place to spin wave determines the ratio of power beingcarried in the electromagnetic channel and the quantum mechanicalexchange channel in the resultant propagating wave at that point and thegroup velocity of the spin wave.

The explanation in the previous paragraph. as mentioned, was made on thebasis that the incident electromagnetic wave is positively circularlypolarized and introduced to a ferromagnetic or other appropriatematerial. having a pair of flat parallel faces. along a direction ofpropagation perpendicular to said faces, i.e., in the z direction. inaddition, the internal magnetic bias H in the material is parallel tothe direction of propagation, and the material is magnetized tosaturation. lf. however. the incident wave is not so directed or if themagnetic bias is directed in some direction other than 2.. then LII willchange and according to expressions (2) and (9) can varyradically fromthe previous value. For example. for It 1 cmf. k 50 emf, )vk' [cf/k andthe expression (2) becomes: tan B 1250111, where 111 is in radians. Achange of 111 from 0 to 1 will alter [3 from 0 to approximately 87.Also, for any given ma- 1 frequencies will be removed at 25 becauseterial at a given magnetic bias. the spatial conversion fromelectromagnetic power can be altered by varying the angle of incidenceto some other value. and in such circumstances It and a respectively ofthe propagating and evanescent spin waves will change from theparticular value obtained using the Z-directed wave. it should be noted.however. that when the angle of incidence and/or d-c field direction arealtered. the simple conditions discussed in the Morgenthaler article donot apply; in particular, spin waves having both an (I and IT (even inthe lossless case) can occur under some conditions and be excited at theinterface. Such surface waves have the property that power is guidedalong or at an angle to the surface and, like the simpler waves, canprovide useful functions, such as delay. switching and/or filtering. Theeffect on power flow due to departures from the special case can be seenupon reference to the mathematical expression previously presented.

Thus, a switch for microwave electromagnetic energy can be made usingthe principles herein disclosed, and such a switch is capable ofeffecting switching from one output to another. as outputs 25 and 26 inFIG. 5, very quickly (of the order of microseconds) with milli-watts ofpower required to effect such switching. The input electromagneticenergy can be introduced to the single crystal YlG material 12 at theinput port 24 through a conductor 15 and removed through either ofoutput conductors l6 and 17 at output ports 26 and 25, respectively.switching the path of power travel within the material 12 being effectedby changing the direction of F! from +111 to b. as before discussed. Anambient magnetic field between the pole piece 21 and 22 of anclectromagnet is provided at a level to establish a magnitude ofinternal bias H at which conversion from electromagnetic power transferto spin wave power transfer can occur. as previously mentioned. Thelevel of H thereby established is such that the wavenumber k of thepropagating spin wave is substantially greater than k,, to enable thelow-power, short-time switching just mentioned. For, as beforediscussed. as l k,,,[3 0, independent of ti], so that the apparatusherein disclosed cannot be used at very low k values. i.e.. much belowabout 5k The discussion in the previous paragraph relates to the switchfunction of the present invention which may also serve a computer memoryelement. It will be noted, however. that wave energy at differentfrequencies will be converted to spin waves having different k/k,,ratios; therefore, according to expressions (2) and (9) the angle [3 isa function of frequency. Thus, with reference to FIG. 5, energy enteringat the port 24 may, for example, pass along the path 18 to be rem ovedat the output port 25. However. only a narr for each narrow band willdiffer from the fi f each other narrow band.

' In order to determine and control time delay of electromagnetic energyintroduced to pass from left to right in the waveguide 65 in FIG. 7, aprobe 76 can be provided within the waveguide to pick up the incomingsignal, which may be in the form of a series of pulses, and the timedelay effected by the material 60 can be determined by relating theincoming pickup by the probe 76 to an outgoing pickup by a probe 77. Theamount of delay can be modified by feeding the signals from the twoprobes to a variable d-c current supply 67 thereby to modify the currentin the coil 66 to render changes 9 in H, as before discussed. Thesignals from the probes 76 and 77 are passed through detectors 7] and75, respectively; and the detected signals are then amplified byamplifier- 72 and 74, the outputs of which are fed to a control logicdevice 73 and thence to the current source 67.

It is possible, therefore, by using a suitable magnetic material andappropriate bias, to achieve a condition where conversion can takeplace. Thus, a YIG rod with suitable dopant added can be biased to avalue at which conversion can take place; and the level of internalmagnetization can be chosen to provide conversion from electromagneticenergy to spin wave energy with no provision for the elastic wave or forconversion of the elastic wave energy, as discussed hereinafter. Inorder to effect conversion at a desired region in the material, thecomposite materials shown at 40, 40', 40" and 40" of FIGS. 8, 9, l0 and11, respectively, may be usedin the waveguide arrangement of FIG. 7.(The z direction in FIGS. 8 and I0 is out of the paper or into thepaper, whereas the z direction in FIGS. 9 and 11, as shown, is to theright.) The composite materials are provided by varying the dopant indifferent regions of the crystal. Thus, for example. radially disposedregions 42,- 42' and 42" of the crystal 40 are doped in such a fashionthat at a given applied field the internal magnetic bias of the film 40will vary from region to region. Similar remarks apply tomulti-transverse layers 43, 43', etc, of FIG. 9, the wedge-shapedregions 41, 41, etc., of FIG, 10 and the multi-axial layers 27, 27',etc., of 40. Thus, a z-directed wave incident upon the delay material ofFIG. 7 will encounter-if crystals similar to 40, 40 and 40" are used-aplurality of transversely disposed regions in which the environmentalparameters vary from region to region and conversion fromelectromagnetic energy to propagating spin wave energy will occur inthat region wherein the environmental parameters are at an appropriatelevel for such conversion. Furthermore. the composite crystal 40" ofFIG. II can include layers of the type disclosed in FIGS. 8, 9 and 10.

A further benefit may be derived from a magnetic material having gradedmaterial parameters. Assume. for example, that the material is graded sothat a uniform external z-directed magnetic field results in an internalbias H which is maximum at the center of the film and decreasestransversely toward the outside edges thereof; that is, with referenceto FIG. 8, the region 42 has maximum z-directed H values and 42' and 42"have respectively lesser values of H at one value of uniform appliedfield. Then electromagnetic energy into the material in FIG. 7 isconverted in the manner previously discussed and will be withdrawn aftera time delay. The length of time delay will depend to a great extentupon the point ofconversion within the film. Energy passing through theregion 42 will convert at a different point along the path of travelthan energy in the region 42, which will, in turn, differ from energy in42". If the gradations vary uniformly from the center outward, forexample, then there will be dispersion of the energy in the material.However, pulses, for example, will have quite uniform time delay anddispersion, even with changes in the magnitude of internal bias H, sincethe effect of such changes will be merely to shift the region throughwhich wave energy of any particular group velocity will travel; and theresultant output pulse formed from the various group velocities in- 10volved will not change appreciably, being effectively a total which willnot be affected as a whole to the extent that the individual componentsthereof are affected. Furthermore, thepoint of conversion will have aneffect on the switching and filtering functions previously discussed.

The external magnetic field provided by the coil 66 should ideally be zthroughout the region occupied by the magnetic material. However, evenif such uniformity exists, the internal bias field H will tend to belower near the faces thereof, as the faces shown at 28 and 29 in FIG.11, than within the interior thereof because of demagnetization due todipole effects. For this reason, it is often desirable to provide amulti-layer material as. for example, the multi-layer device 40" inwhich the dopant and amounts of dopant differ from layer to layer tocounteract to some extent the dipole effect and also to provide theabrupt discontinuity as a series of smaller discontinuities. Thus, thematerial 40" can be used to provide a particular profile of internalbias H in the z direction and can be used, for example, to overcome tosome extent the demagnetizing effect of dipole action. Furthermore,whereas changes in effective internal field strength, effected bychanges in the external field or by effectively reducing the distancebetween field lines, are completely governed by Maxwells equations,similar changes effected by modifying the dopant are not so governed,insofar as the magnetic anisotropy is involved. Even without anisotropy,a field produced by a magnetization vector with non-zero divergence isnot a Laplacian field; the latter is often not the most appropriate typeof field. The outer layers 27 and 27" may, for present purposes. bedoped with larger amounts of, say, gallium than the inner layers 27 and27". the larger amounts of gallium being effective to increase H in thelayers 27 and 27" for a given applied field. An appropriate amount ofdopant can be pro vided in each layer, using the teachings in theAnderson et al. article, to give the required profile of the bias fieldH.

The discussion with reference to FIGS. 5 through 11 relates tosituations where the magnetic bias field H and the wave energy aregenerally :-directed, the angle 11; being typically less than 1 from thez direction. It can be seen from the mathematical expressions thatsimilar results obtained when the external field is oriented near adirection 'rr/Z radians from the z-directed field previously discussed.However, whereas an applied field of 2000 gauss, typically will berequired to provide a directed internal field H of about 300 gauss (forexample, to convert a 1,000 Hz. electromagnetic signal to a spin wave),a 300 gauss external field directed in the plane of the material (inwhich m is vr I0 radians/- sec) will result in an internal field H ofabout 300 gauss. However, to shift the field from +tl1 to -11], forexample, requires more power when the field generally is directed in theplane of the material than when the field is directed generally in the zdirection. (It should be here noted that the crystals discussed hereinmay measure typically a few spin wave lengths in the z-direction buttens of wavelengths in cross dimensions.)

As previously mentioned, when wave energy within the magnetic materialis at or near the value k, (values of k within the range 11 /2 to .n 3are considered near k for present purposes), appreciable time delay canbe obtained. In the absence of loss a theoretical delay time of theorder of 2.5 microseconds in 10"cm.

is possible in YIG, thus rendering the present invention useful, also,in connection with thin film devices.

In the prior discussion it'is shown that the group velocity of thepropagating wave is quite sensitive to changes in the internal magneticbias field H, which, in turn, is a function of the external field. It ispossible, therefore, by noting changes in the group velocity, timedelay, to relate such changes to the changes in the external fieldthereby to determine the magnitude of any changes in the external field.The circuitry disclosed in FIG. 7 can thus be used to provide a measureof the external field. j

References to the wavenumber k in the present specification relates inall instances to the propagation constant of the spin wave, and thedecay rate relates only to the reactive or evanescent decay; it shouldbe noted, however, that a propagating spin wave in a non-ideal materialhas a decay rate, and the evanescent wave has a wavenumber. Aspreviously noted, there can exist certain spin waves (even in thelossless case) having both an and I: T he effect of loss on these wavesis to modify the iand lTvalues so as to produceattenuation in thedirection of energy propagation. Also, the term electromagnetic is, bynecessity, used in more than one context herein; an attempt has beenmade, however, at all times to make clear in what way the term is used.In air the power transfer and energy form of the propagating wave areboth electromagnetic, but within the magnetic material described thespin wave power transfer mechanism is exchange and electromagnetic.

In the material the small signal energy density in both the propagatingspin wave and the evanescent wave contain the factors shown in thefollowing mathematical analysis:

+ V: ;t,, A sfmxchange) 4,, N m, In,

(magnetic anisotropy) (summation over a repeated subscript is implied),where the terms electric, etc., in parentheses designate the type ofenergy represented by the terms immediately preceding the parentheticalterm. The elements in the immediately preceding analysis represent thefollowing: W is the small signal (s.s.) energy density (joules/m);? isthe s.s. electric field (volts/m); e is the permittivity of thematerial; i.e., the permittivity in air (s 1/3671" farads/meter) timesthe dielectric constant; ,u.,, is the permeability of free space (4,uhu.10 henries per meter); IT is the magnetic field (amperes/m); H is theinternal d-c magnetic field (amperes/m); M is the saturationmagnetization (amperes/m); m is the s.s. magnetization (arnperes/m.); A

is the exchange constant (m and N are dimensionless constants whichdepend upon orientation of the crystal relative to the internal d-cfield H, the term N m,- m,- upon expansion taking the form N, "m 2N "m mN "m where: m "1,, "225 m and Further, the s.s. power flux (watts/m isrepresented y H X li a plurality of output transmission lines and toswitch output power from one to the other of these outputs as in a stripline or the like, or to provide a microwave computer function suchbinary switching.

The discussion to follow is concerned primarily with the concept ofdoping the YIG crystal to provide material parameters or characteristicswhich, in combination with the external magnetic bias field H, providean internal bias field H profile of a predetermined shape or contour.Thus, the magnetic field profiles shown in FIGS. 2B,- 2C and2D in saidpatent 3,530,302 in the rod 1 of FIG. 6A thereof can be provided. Itwill be appreciated, also, that other profiles can be furnished asneeded, a magnon tunnel transducer particularly useful at frequenciesabove S-band (3.000 MH3) and into X-band and higher. Such transducershave graded values of saturation magnetization 411'M as discussedpreviously herein and as now discussed in connection with the layers 27,27. 27", 27 in FIG. 11. For example, the layers 27 and 27" can each bethe order of -100 microns thick and in a uniform z-directed externalfield H applied to a device having graded susceptibility could provide agradient in the saturation magnetization of say 50,000 oe/cm. Thesaturation magnetization 417M, at the input end 28 of the device 40"could be 1 750 oersteds and the saturation magnetization 41rM at theright side of the layer27 could be -l,250 oersteds, for example. Thelayer27".' on the other hand. could have a saturation magnetization41'rM of -l,750 oersteds at the end 29 and -l,-250 oersteds at theinside and thereof, and the layers 27' and 27" can have uniformsaturation magnetization 41rM -l,250 throughout. The conversion fromphoton-to-magnon-to-phonon would occur in-the layer 27 and reconversionin the layer 27" of such device. The layers 27 and 27" in the device 40"can be epitaxially grown on the respective ends of a single bulk crystalin the manner hereinafter discussed. Input and output could be effectedby the fine-wire coupling shown in said US. Pat. No. 3,530,302 or by thearrangement shown in FIG. 7 hereof. The layer 27 serves to transduceinput photons to magnons which rapidly tunnel into the epitaxial layer27 to a region at which tunneling yields to propagation, after whichconversion to phonons occurs. The time required for tunneling andconversion is inversely proportional to the magnitude of the gradient ofthe effective field within the layer 27,

which in this instance, is about equal'to the magnitude X 4 -54 I430oersteds .m-M 5 R90 oersteds X 4 'M E 350 ocrstcds'.

and that the magnetizing field H used is the order of 500 to 2,000gauss.

A number of methods may be employed to grow gradedparameter crystals forthe above purposes; two such methods are described in this and thefollowing paragraph and they relate respectively to thin films and bulkcrystals. The method described in the instant paragraph is that ofchemical vapor deposition for preparation of thin films. Single crystalYIG is deposited ac' cording to a chemical reaction occurring at thevaporcrystal interface in which gaseous yttrium and iron ha lides reactwith water vapor and/or oxygen to produce Y;,Fe,-,O The metal-halidevapors needed for these reactions are produced in a furnace by passing ahalide (e.g. chlorine) gas over hot metal powder of iron and yttrium inthe first zone of a furnace. deposition upon a substrate occurring at asecond zone in the furnace The furnace is controlled to providetypically a temperature l,l50C in the halide generation zone and l,lO()to l,4()0C in the chemical vapor deposition zone. A pressure of two toten Torr was used in actual apparatus; a gas flow of cc/minutes; and therun times were to a 120 minutes. Gallium to give the graded parametercrystals can be added as gallium halide gas in the chemical vapordeposition zone.

Bulk crystals have been grown by the top-seeded method to lengths of twocentimeters and up to four centimeters in diameter in anon-stoichiometric melt using BaO-B O;, as the solvent. Melts containingfrom eighteen to 26 weight percent YlG give crystallization of thegarnet phase in a temperature range between l,O0()C and l,25()C. Galliumcan be introduced into the melt during the growth process, but othermeans for introducing the gallium may be used. Thus, for example, a YlGcrystal grown to size without doping and polished can be dipped at eachend. into a solution of Ga- YlG at a temperature of l,l()0C for 15minutes, and the gallium in the solution will chemically replace some ofthe iron in the YlG at the ends to give a spatially non-uniform thinfilm at each of the two ends, (See two journal articles by R. C. Linaresentitled Substitution of Aluminum and Gallium in Single-Crystal Yttriumlron Garnets," Journal of American Ceramic Society, pp. 6878, Vol. 48,No. 2, February, 1965, and Growth of Single'Crystal Garnets by aModified Pulling Technique. pp. 433-4, Journal ofApplied Physics, Vol.35, No. 2, February, 1964). The graded film, thereby formed, providesthe quantum mechanical tunneling properties of a single-crystal filmwhich can act to transduce the photon input energy converting that inputto the magnon and phonon forms before mentioned. The graded film, whichcan typically be -5()l()() microns thick is made up of a series of suchcontiguous layers (by repetitive dipping into Ga-YlG solutions havingdifferent ratios of Ga and Fe) one or more microns thick which vary fromlayer to layer to give. essentially, a continuous change inmagnetization from one end to the other. The graded film can be thelayers 27 and 27" in FIG. 11. Similar effects can be provided byappropriate masking of the crystal to produce the various configurationshown in FIGS. 9 and 10. It will be appreciated that the layers 27 and27 and the layers in several of the other figures can be grown by thechemical deposition method previously discussed. It will be furtherappreciated that the internal magnetic bias field H, which is a functionoffi plus the material parameters, can be electronically tuned byeffecting changes in l-T.

The term susceptibility as used herein is an extension of the Poldermagnetic susceptibility tensor generalized to include spatial, as wellas temporal, dispersion in its components. This tensor includes theeffects of coupling among spin waves, elastic waves and theelectromagnetic fields. A system comprising electromagnetic power, spinwave power, and elastic wave power can be viewed as an equivalentelectromagnetic system provided the concept of the Polder magneticsusceptibility tensor is extended to include spatial as well as temporaldispersion in its components. This dispersion accounts for the couplingamong spin waves, elastic waves and electromagnetic waves.

Further modifications of the invention herein dis closed will occur topersons skilled in the art and all such modifications are deemed to bewithin the spirit and scope of the invention as defined in the appendedclaims.

What is claimed is:

1. Apparatus for processing electromagnetic wave energy, that comprises,a magnetic material adapted to receive the electromagnetic wave energy,the material being one that will support propagating spin wave energyand propagating elastic wave energy and allow interchange of energytherebetween, the material being one in which the electromagnetic waveenergy can be converted to propagating spin wave energy and propagatingelastic wave energy, the material being further adapted to receive anapplied magnetic field which in combination with the susceptibility ofthe material pro vides a graded internal magnetic bias field to effectspatial conversion from electromagnetic wave energy to eitherpropagating spin wave energy or propagating elastic wave energy, thematerial being treated to provide a predetermined grading of saidsusceptibility thereby to present a predetermined profile of internalmagnetic bias field to the wave energy, means for introducing saidapplied magnetic field to the material, input means for introducingelectromagnetic wave energy to the material, and means for removingelectromagnetic wave energy from the material.

2. Apparatus as claimed in claim 1 that further includes means fordetermining a level of applied magnetic field required to effect saidconversion and means for adjusting the level of the applied magneticfield.

* l= l l

1. Apparatus for processing electromagnetic wave energy, that comprises,a magnetic material adapted to receive the electromagnetic wave energy,the material being one that will support propagating spin wave energyand propagating elastic wave energy and allow interchange of energytherebetween, the material being one in which the electromagnetic waveenergy can be converted to propagating spin wave energy and propagatingelastic wave energy, the material being further adapted to receive anapplied magnetic field which in combination with the susceptibility ofthe material provides a graded internal magnetic bias field to effectspatial conversion from electromagnetic wave energy to eitherpropagating spin wave energy or propagating elastic wave energy, thematerial being treated to provide a predetermined grading of saidsusceptibility thereby to present a predetermined profile of internalmagnetic bias field to the wave energy, means for introducing saidapplied magnetic field to the material, input means for introducingelectromagnetic wave energy to the material, and means for removingelectromagnetic wave energy from the material.
 2. Apparatus as claimedin claim 1 that further includes means for determining a level ofapplied magnetic field required to effect said conversion and means foradjusting the level of the applied magnetic field.